Many different types of work performed at sea or on the ocean floor require vessels, barges or other floating platforms that need to hold station in open sea or accurately follow pre-determined tracks relative to the ocean floor. Projects requiring such vessels include offshore drilling, subsea pipelay and cable lay, subsea construction, salvage and recovery, oceanographic research, etc.
The vessels, barges and floating structures used for such projects are often equipped with multiple anchors and winches, commonly referred to as anchor mooring systems. They require support of anchor handling vessels to position the anchors at pre-determined locations and move the anchors as needed.
As oil and gas exploration is extending farther and farther offshore from land, more and more of these projects are taking place in water depth sufficiently great that it is impractical, sometimes impossible to use anchor mooring systems. Even in some shallow water areas, the use of anchor mooring systems may be prohibited, for instance, due to the presence of coral reefs or in locations where there already are multiple pipe lines and cables on the ocean floor and the use of anchors could damage the coral reefs or break existing pipe lines and cables.
For such applications, vessels, barges and floating structures equipped with dynamic positioning systems are used. These vessels are equipped with multiple thrusters operated by computers to adjust and maintain the heading and the positioning of the vessel against environmental forces of current, wind and waves. The thrusters include propellers that are operated to create thrust forces that are applied to the vessel for movement of the vessel in desired directions. In a tunnel thruster, the propeller is located in a tunnel that extends transversely through the vessel below its water line, usually near the bow or the stern of the vessel. Tunnel thrusters are used in is combination with the conventional fixed axis propulsive propellers at the stern of the vessel to adjust and to maintain the heading in the position of the vessel over a defined spot on the sea floor.
Retractable and steerable thrusters are also known in the context of dynamically positioned ships and other floating facilities. Whereas tunnel thrusters generally apply thrust reaction forces to a vessel only in one or the other of two opposite directions transversely of the vessel hull, steerable thrusters apply thrust reaction forces in any desired horizontal direction relative to the hull. For that reason, steerable thrusters are increasingly preferred for vessels, barges and floating structures requiring station keeping in open waters without using anchors.
Most steerable thrusters are installed inside the hull, extending through the bottom of the vessel. They are powered by electric motors and the electrical power is provided by large generator sets installed inside machinery rooms of the vessel. These thrusters and power systems are permanent fixtures and completely integrated within the vessel through electrical power distribution, control power, cooling water systems, fuel systems, structural support, etc.
A portable positioning system with portable thrusters, self-contained power units and a dedicated control system has long been needed, where the thrusters, power units and controls are not integral with any of the ships systems or integral with the hull of the ship and allow easy attachment to a mono hull or multi-hull ship and easy removal when the system is no longer required for that vessel but can be installed on a different vessel for another application.
Additionally, a need has existed for a modular system that can easily be increased or reduced in overall size and capacity to suit individual project application requirements and for adaptation to different size vessels, barges or other floating structures.
Additionally, a need has existed for a fully packaged, self-contained system that is fully integrated, factory tested and Class approved before installation on the ship, allowing vessel upgrades to dynamic positioning capability within just a few days and at minimal cost.
Additionally, a need has existed for a system which is easy to service at sea allowing minimal down time without the need for a shipyard or dry dock, allowing the vessel to continue operating at its work location without interruption, hence increasing the profitability of the operation.
This system meaningfully addresses the above needs in the context of dynamic positioning of vessels, barges and other floating structures.